The invention concerns a method and associated device for ion thinning of a sample in a sample region of a transmission electron microscope having an objective lens and comprising an ion source for the production of an ion beam, an ion lens, a secondary electron detector for the production of an ion scan secondary electron image of the sample surface with the assistance of the secondary electrons released during scanned ion irradiation and for the positioning of the ion beam, via the ion scan secondary electron image, onto a particular location of the sample.
Samples are prepared for high resolution transmission electron microscopy by being pre-thinned in a conventional manner, using mechanical or chemical procedures, to an initial thickness of ca. 10 .mu.m. The samples are subsequently ion thinned through bombardment with an ion beam with the ion beam at as flat an angle as possible with respect to the sample surface until a small hole of approximately 100-200 .mu.m in diameter is produced in the middle of the sample. The sample is then sufficiently thin within a wedge-shaped region in the edge portion of the hole and is transparent for fast electrons in excess of 100 keV.
For high resolution transmission electron microscopy the amount of inelastic scattering of the electrons in the sample is low, so that it is necessary for the sample to be sufficiently thin. A high lateral resolution of approximately 1.5 nm, corresponding to an electromicroscopic enlargement of 100,000, can be achieved beginning with a thickness of approximately 100 nm. In highest resolution transmission electron microscopy with which lateral resolutions of up to 0.15 nm are achieved, corresponding to magnification factors of 1 million, it is necessary for the sample to be thinned to a thickness of less than 10 nm. In addition to high resolution electron microscopy, very thin object locations free from reactive layers, contamination layers and amorphous layers damaged by ion thinning, are also required in electron holography.
The usual ion thinning procedures utilized for sample preparation in transmission electron microscopy work in a substantially "blind" manner. Whether or not a desired object feature is properly enhanced with good quality through preparation is more or less a question of luck. Devices of prior art have therefore been proposed with which a sample in a transmission electron microscope sample holder can be thinned in an external etching device under simultaneous observation.
The publication Gatan Inc., 6678 Owens Drive, Pleasanton, Calif. 94566 USA product specification "Precision Ion Milling System" (PIMS), model 645, June 1987, has proposed imaging of the released secondary electrons for this purpose. The publications H. Bach, Bosch Technische Berichte 1, 1964, 10-13 and F. Nagata et al., Proc. 41 st. Confer. JSEM. 1985, 133, have proposed observing the sample for ion thinning in the first imaging plane of a transmission electron microscope in transmission mode.
These conventional etching devices have the disadvantage that the resolution which can be achieved for evaluation of the sample quality is insufficient and that, after ion thinning, it is necessary to introduce the sample along with the sample holder into the field of the objective lens for high resolution observation of the sample. The thin sample thereby reacts with air and with residual gas causing undesirable reaction and contamination layers.
It is furthermore disadvantageous that a plurality of transfers are normally necessary between high resolution observation and additional thinning in order to achieve the desired layer thickness. It is thereby not only difficult to once more locate the object position for renewed observation, which can be extremely difficult for high lateral resolution, but the high resolution transmission electron microscopic observation also causes disturbing contaminating layers due to the high energy electron irradiation, in particular from hydrocarbons. It can therefore be necessary to prepare a new sample location through renewed thinning at another location.
Another problem which occurs during high resolution transmission electron microscopy is associated with the fact that the objective lens cannot be arbitrarily switched-on and off. The objective lens produces the magnetic field necessary for high resolution at the location of the sample of ca. 1 to 2 T and effects a constant magnification of 100 to 200. When switching-on the objective lens, drifts occur due to various influences such as current stability, heat expansion and heat equilibration effects as well as other causes which require a time duration of two hours or longer to damp to a level sufficiently stable for observation. It is therefore not possible to quickly change from high resolution observation with switched-on objective lens having a magnification of approximately 1,000,000 to low, long focal length magnification with the objective lens switched-off with, if appropriate, switching-on a mini or intermediate lens producing no magnetic field in the objective plane having ca. 10,000-fold magnification. On the other hand, due to the very limited amount of space particularly in the region of the objective lens, it is not possible to move a sample in the vacuum of the transmission electron microscope out of the first imaging plane position into the objective plane. In order to do this, the sample must be passed through a vacuum lock.
DE 29507225 U1 proposes an ion beam preparation device for electron microscopy which facilitates ion thinning during simultaneous electron microscopic observation. A high resolution observation is however not possible with the configuration proposed therein, since no magnetic field is present at the location of the sample due to the lack of an objective lens located therein. The electron energy is also low. The sample is not disposed in the sample region of the objective lens of a transmission electron microscope.
JP-(A) 6-231719 proposes a preparation method for the preparation of cuts in semiconductors with which a sample location having a passive protective layer of ca. 100 nm thickness is thinned in the objective plane of the objective lens of a transmission electron microscope using a fast-ion-beam-ion source of 30 keV. The high energy of the ions substantially reduces problems with regard to working separations but causes radiation damage and the large amount of material removal leads to deposits on the lenses and therefore to imaging distortions. Nor is it possible with the apparatus proposed in this publication to thin the sample under high resolution conditions, i.e. with magnifications in excess of 10,000 or 100,000, since the ion beam is not incident on the sample when the objective lens is switchedon. The publication therefore proposes switching back and forth between ion thinning and higher resolution which is associated with the above mentioned disadvantages.
Those of average skill in the art have believed that it is necessary to carry out ion thinning with the objective lens switched-off to allow the ion beam to be incident on the sample location. The disadvantages associated therewith were accepted up to this point in time.